My wife was not sure whether she wanted a second child. I think the thing that made her want to do so was when I asked her if she could imagine life without her sisters.
The first week two that Toodles was home, JD, for the most part, ignored her. As he has come to terms that she is not going anywhere, he has warmed up to her and then some.
He first began trying to play with her while I put her on her small play mat for tummy-time. JD would come over and play with the trinkets on the soft poles that arch over the mat. Then he began to point to her body and identify eyes, nose, mouth, ears (though I have to make sure he keeps his fingers out of those places!) Then he would pat her head and her belly. Finally, he asked to hold her, so I had him sit on my lap and then brought her on top of his.
My favorite moment so far was the other day in the car. I looked in my mirror and saw JD's hand in her car seat. He was looking down at her. Though I couldn't see inside her rear-facing car seat, I think that he was holding her hand while she slept.